Stir forming apparatus and method

ABSTRACT

An apparatus and method for forming a workpiece to a desired, non-planar configuration are provided. At least one friction stir forming tool, having a shoulder and a pin, is used to urge the workpiece to the desired configuration and friction stir form the workpiece. The forming tool can urge the workpiece against a contour surface of a die or a shoulder that is opposite the structural member from the tool. Thus, the forming tool plasticizes a portion of the workpiece and urges the workpiece to the desired configuration. In addition, the material properties of the workpiece can be improved by the friction stir processing.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to the forming of structural members and,more particularly, to the forming of structural members in conjunctionwith friction stir processing.

2) Description of Related Art

The forming of structural members to desired shapes can be accomplishedby forging, stamping, bending, machining, and the like. For example, apreform or blank comprising a flat sheet of metal such as steel,aluminum, titanium, or alloys thereof can be heated in a forge andhammered to the desired shape of the member. Alternatively, a mechanicalor hydraulic press can be used to stamp or forge the structural memberto the desired shape, e.g., between two opposing dies that defineforming surfaces that correspond to the desired contour of the member.Such forming methods are typically quicker and less expensive thancasting molten material in the desired shape, and complex shapes can beimparted to the member, such as curves, angles, and the like. However,depending on the particular material of the preform, cracks or weakenedportions may result from excessive forming, e.g., the formation of sharpangles or other significant deformations. Therefore, the formation of aparticular complex structural member may require that multipleelementary members are formed and then joined by welding or otherjoining methods. Further, subsequent material processing, e.g., heattreatments such as solution heat treating, aging, and quenching, may berequired to achieve desired properties in the formed member. Theseadditional operations add to the manufacturing time and cost of thestructural members.

The structural members can also be treated before being formed to thedesired shape. For example, according to one proposed method, a preformis supported in a flat configuration, and a friction stir welding toolis inserted into the preform and rotated to plasticize the material ofthe preform. The friction stir-welding tool is then moved continuouslyover the surface of the preform so that part of the preform is frictionstir processed. The preform is then formed, for example, by forging,stamping, bending, or the like, to the desired shape of the member. Suchfriction stir processing refines the granular structure of the preform,improving the material properties of the structural member. However,this process is also time-consuming and adds to the cost of thestructural member.

Thus, there exists a need for an improved apparatus and process forforming structural members. The process should be capable of formingvarious configurations of structural members, including those withcomplex geometric configurations. Further, the process should result instructural members having desired material properties.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for forming aworkpiece to a desired, non-planar configuration with at least onefriction stir forming tool. The forming tool plasticizes a portion ofthe workpiece and urges the workpiece to the desired configuration,thereby refining the grain structure of the workpiece. Thus, theworkpiece can be formed to any desired configuration, including complexshapes. In addition, the material properties of the workpiece can beimproved.

According to one embodiment of the present invention, the apparatusincludes a die defining a contour surface such as a dome shape thatcorresponds to the desired configuration of the workpiece. The die canalso define a detail feature corresponding to a feature that is to beformed on the workpiece. A friction stir forming tool, including ashoulder and a rotatable pin extending therefrom, is configured to urgethe shoulder and pin toward the contour surface of the die. Thus, theshoulder urges the workpiece against the die and to the desiredconfiguration, and the pin at least partially penetrates the workpieceand rotates to form a friction stir formed region defined by a refinedgranular structure. The apparatus also includes an actuator foradjusting the friction stir forming tool along a predefined pathcorresponding to the contour surface of the die, e.g., in at least twolinear directions and about at least two axes of rotation. The die canalso be rotated relative to the forming tool.

According to another embodiment of the present invention, the apparatusincludes first and second friction stir forming tools in an opposingconfiguration so that the shoulders are directed generally inward toreceive a workpiece therebetween. The forming tools are adjusted by oneor more actuators in an inward direction against opposite sides of theworkpiece so that the pins at least partially penetrate the workpiece.The pins are rotated to form a friction stir formed region in theworkpiece, and the actuator adjusts the friction stir forming toolsalong a predefined path corresponding to the desired, non-planarconfiguration of the workpiece so that the shoulders urge the workpieceto the desired configuration.

The present invention also provides a method for forming a workpiece.The workpiece is supported at least partially against the die, and theshoulder of the forming tool is urged against the workpiece. The pin atleast partially penetrates the workpiece and the workpiece is bentagainst the contour surface. The pin of the tool is rotated in theworkpiece to plasticize a portion of the workpiece, thereby forming afriction stir formed region having a refined grain structure. The toolis adjusted in a predefined path that corresponds to the contour surfaceof the die so that the shoulder urges the workpiece to the desired,non-planar configuration. An entire surface of the workpiece can beplasticized, thereby refining the grain structure of the workpiece overthe surface. Further, two or more structural members can be provided andjoined to form the workpiece.

A method according to another embodiment of the present inventionincludes forming the workpiece by first and second friction stir formingtools, which are configured in an opposing configuration so that theshoulders are directed generally inward toward the workpiece supportedtherebetween. The shoulders of the first and second tools are urgedagainst the respective sides of the workpiece, and the pins of the firstand second tools at least partially penetrate the respective sides. Thepins are rotated to plasticize opposite portions of the workpiece,thereby forming opposite friction stir formed regions having refinedgrain structure in the workpiece. The tools are also adjusted incorresponding predefined paths so that the shoulders urge the workpieceto the desired, non-planar configuration.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a perspective view illustrating an apparatus for forming aworkpiece according one embodiment of the present invention;

FIG. 2 is a section view illustrating the apparatus of FIG. 1, shownwith the workpiece partially formed;

FIG. 3 is a section view illustrating an apparatus for forming aworkpiece according to another embodiment of the present invention;

FIG. 4 is an elevation view illustrating an apparatus for forming aworkpiece according to yet another embodiment of the present invention;

FIG. 5 is an elevation view illustrating the apparatus of FIG. 4, shownwith the workpiece partially formed; and

FIG. 6 is an elevation view illustrating an apparatus for forming aworkpiece according to still another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, this invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Referring now to the drawings, and in particular to FIG. 1, there isshown an apparatus 10 for forming a workpiece 100 to a desiredconfiguration. The workpieces 100 can be formed of metal, such as steel,aluminum, titanium, or alloys thereof, polymers, and other materialsthat are compatible with friction stir processing. The workpieces 100can be used for a variety of applications including, but not limited to,structural panels, brackets, tubes, caps, and the like. For example, adome-shaped workpiece 100 and a tubular workpiece 110 (FIG. 3) can beformed and joined to create a closed or partially closed vessel. Theworkpieces 100 can be used as structural members for vehicles for theaerospace, aeronautical, marine, and automobile industries, or for otherstructures such as pressure vessels, weaponry, and the like.

The apparatus 10 includes a die 20, which defines a contour surface 22that corresponds to the desired, non-planar configuration of theworkpiece 100. The contour surface 22 of the die 20 shown in FIG. 1 isdome-shaped, but the contour surface 22 can alternatively define othershapes including complex curves, angles, and the like. Preferably, thecontour surface 22 is non-planar, i.e., the surface 22 defines one ormore curves, angles, or other non-planar features. For example, anon-planar surface can be defined by two or more planar portions thatare disposed at an angle. A non-planar surface can also be formed bydisposing ridges, channels, or other features on a planar die. The die20 is secured to a base 30 or frame, which can be configured to rotateor otherwise move the die 20. The die 20 partially receives theworkpiece 100, e.g., so that the workpiece 100 is in contact with aportion of the contour surface 22. A bolt 24 or other connection devicecan be used to secure the workpiece 100 to the contour surface 22 of thedie 20.

The apparatus 10 also includes a friction stir forming tool 40 with ashoulder 42 and a pin 44. An actuator 50 is configured to rotate thetool 40 and adjust the tool 40 relative to the die 20 and the workpiece100. Thus, the actuator 50 can advance the rotating tool 40 toward theworkpiece 100 and at least partially penetrate a first side 102 of theworkpiece 100 with the pin 44. The actuator 50 is also configured toadjust the tool 40 along a predetermined path so that the rotating pin44 is moved through the workpiece 100. For example, the actuator 50 canbe connected to a linkage or frame 52 that supports and/or moves theactuator 50. In particular, the actuator 50 and/or the frame 52 can be acomputer numeric controlled (CNC) device, which can be programmable, asconventionally used for controlling a machining head.

As the rotating pin 44 is moved through the workpiece 100, the pin 44frictionally heats and plasticizes a region of the workpiece 100. Theplasticized region then cools and hardens to form a friction stir formedregion 114 having a refined granular structure, as shown in FIG. 2.Thus, the effect on the material properties of the workpiece 100 thatresults from the friction stir processing operation of the presentinvention is similar to or the same as that which results from frictionstir welding, as described in U.S. Pat. No. 5,460,317 to Thomas, et al.,the entirety of which is incorporated herein by reference. As a resultof the refined granular structure of the workpiece 100, which generallyresults from friction stir forming, the material properties of theworkpiece 100 can be improved, for example, by increasing the strengthand corrosion resistance of the workpiece 100.

The friction stir forming tool 40 and/or the die 20 can be adjustable tomove the tool 40 through the workpiece 100. For example, as indicated inFIG. 1, the die 20 is connected to the base 30 via a rotation device 32that is configured to rotate the die 20 and the workpiece 100 relativeto the base 30, either manually or by an actuator (not shown). Further,the friction stir forming tool 40 is configured to be adjusted by theactuator 50 through an arc corresponding to the contour of the die 20,as indicated in FIG. 2. That is, the tool 40 is moved through a curvedpath and rotated about an axis perpendicular to the longitudinaldirection of the pin 44, so that the pin 44 is moved through theworkpiece 100. Thus, the friction stir forming tool 40 and the die 20can be adjusted so that the pin 44 is moved through the entire surface102 of the workpiece 100. For example, the friction stir forming tool 40can be adjusted radially outward from the center of the workpiece 100toward an edge of the workpiece 100 while the die 20 is rotated, therebyadjusting the pin 44 through a spiraling path through the workpiece 100.

In other embodiments, the friction stir forming tool 40 and the die 20can move in other manners relative to one another. For example, the die20 can be held stationary while the forming tool 40 is moved throughoutthe surface 102 of the workpiece 100, and the forming tool 40 can berotated about multiple axes so that the shoulder 42 is keptsubstantially parallel to the contour surface 22 of the die 20 and/orthe surface 102 of the workpiece 100. In any case, the pin 44 can beperpendicular to the shoulder 42 so that the pin 44 is keptsubstantially perpendicular to the surface of the workpiece 100 as theshoulder 42 is urged thereagainst, i.e., the pin 44 is perpendicular toa plane tangential to the contour surface 22 at a point collinear withthe pin 44.

The pin 44 can be long enough to extend through the entire thickness ofthe workpiece 100. Alternatively, the pin 44 can be shorter than thethickness of the workpiece 100, for example, about half the thickness ofthe workpiece 100. Further, the pin 44 can be adjustable relative to theshoulder 42 so that the pin 44 can be extended, as shown in FIG. 2, andretracted, as shown in FIG. 1. Thus, the tool 40 can be used to processworkpieces 100 of different thicknesses. In addition, the pin 44 can beadjusted during processing of the workpiece 100 so that the pin 44 canpenetrate different portions of the workpiece 100 at different depths.For example, if the workpiece 100 is nonuniform in thickness, the pin 44can be extended while processing the thicker portions of the workpiece100 and retracted while processing the thinner portions of the workpiece100. The pin 44 can also be extended from the shoulder 42 and into theworkpiece 100 during the start of each processing operation and/orretracted from the shoulder 42 and, hence, from the workpiece 100 at theend of each processing operation.

As the pin 44 is moved throughout the workpiece 100, the shoulder 42 isurged against the workpiece 100 and urges the workpiece 100 toward thecontour surface 22 and to the desired configuration or the workpiece100. Thus, the workpiece 100 can begin in a flat configuration and canbe bent or otherwise formed to the desired shape, as illustrated in FIG.2. Further, although the thickness of the workpiece 100 is shown to beuniform in FIG. 2, the shoulder 42 can urge the plasticized materialfrom the processed portions of the workpiece 100, thereby formingnonuniformities in the workpiece 100, or forming a uniform workpiece 100that is thinner throughout. The predetermined path can pass through theentire surface 102 of the workpiece 100, i.e., so that the granularstructure of substantially the entire surface 102 is refined.Alternatively, the pin 44 can be passed through only select portions ofthe workpiece 100, for example, at incrementally spaced locations or atparticular portions where grain refinement is desired in the workpiece100. In addition, detail features 26 such as grooves, ridges, apertures,knobs, and the like can be provided on the contour surface 22 so that,as the workpiece 100 is formed against the die 20, a correspondingdetail is formed in the workpiece 100. The detail feature 26 shown inFIGS. 1 and 2 is a groove extending circumferentially around the die 20,which causes a corresponding ridge to be formed in the workpiece 100.

A forming apparatus 10 a according to the present invention can also beused to join the workpiece 100 to another member 110. For example, asshown in FIG. 3, the die 20 defines a dome-shaped contour similar to theone shown in FIG. 1. However, an extension 28 is disposed between thebase 30 and the die 20 so that an additional member 110, such as a tube,can be disposed proximate to the workpiece 100. The tube 110 issupported by a support member 34 such that the tube 110 is positionedadjacent the workpiece 100 when the workpiece 100 is formed to thedesired configuration so that an interface 112 is defined therebetween.Clamping members 36 can also be provided for securing the tube 110 tothe support member 34. Further, the tube 110 and the die 20 aresupported by the rotation device 32 and are rotatable relative to thefriction stir forming tool 40. As shown in FIG. 3, the pin 44 of thefriction stir forming tool 40 can be inserted at the interface 112 ofthe workpiece 100 and the tube 110, and the workpiece 100 and the tube110 can be rotated by the rotation device 32 so that the pin 44 isadjusted through a circumferential path defined by the interface 112.The friction stir forming tool 40 frictionally heats and plasticizesmaterial of the tube 110 and workpiece 100 at the interface 112, therebyforming a friction stir weld joint at the interface 112. Thus, after theworkpiece 100 is formed, the workpiece 100 can be joined to theadditional member 110, and the joining can be performed in the sameapparatus 10 a as the forming operation.

FIGS. 4 and 5 illustrate another forming apparatus 10 b of the presentinvention in which the workpiece 100 is formed by two opposed frictionstir forming tools 40 a, 40 b. Each of the tools 40 a, 40 b can besimilar to the tool 40 described above, including a shoulder 42 a, 42 band a pin 44 a, 44 b extending therefrom. The tools 40 a, 40 b arepositioned opposite the workpiece 100 so that the shoulders 42 a, 42 bare directed inward toward opposite sides 102, 104 of the workpiece 100.The workpiece 100 is supported by clamping members 38 or other supportdevices, which are secured to a base or frame 39. For example, theclamps 38 can be mechanical or hydraulic clamps that secure theworkpiece 100 in position. Actuators 50 a, 50 b, which are connected toa linkage or frame, rotate the tools 40 a, 40 b and adjust the tools 40a, 40 b along predetermined paths. Thus, the actuators 50 a, 50 b rotatethe pins 44 a, 44 b and urge the tools 40 a, 40 b inward. The pins 44 a,44 b at least partially penetrate the opposite sides 102, 104 of theworkpiece 100, and the actuators 50 a, 50 b move the tools 40 a, 40 balong the predetermined path so that the regions of the workpiece 100are plasticized and the shoulders 42 a, 42 b of the tools 40 a, 40 burge the workpiece 100 to the desired configuration. For example, theactuators 50 a, 50 b can translate along the length of the workpiece100, while maintaining the shoulders 42 a, 42 b in a parallelconfiguration. The actuators 50 a, 50 b can also rotate about an axisperpendicular to the longitudinal axis of the pins 44 a, 44 b, e.g.,between the positions shown in FIGS. 4 and 5, thereby urging theworkpiece 100 to a curved configuration. More complex shapes, includingthree-dimensionally curved surfaces, can also be formed in this manner,e.g., by adjusting one or both of the friction stir forming tools 40 a,40 b in two or more linear directions and about two or more axes ofrotation. In the formed configuration, the workpiece 100 definesfriction stir formed regions 114 that are defined by a refined granularstructure.

As illustrated in FIG. 4, the pins 44 a, 44 b of the friction stirforming tools 40 a, 40 b can have a length that is about half of thethickness of the workpiece 100 or less so that the pins 44 a, 44 b donot contact one another when urged into the workpiece 100 from oppositesides. Each pin 44 a, 44 b can also be adjustable relative to therespective shoulder 42 a, 42 b, so that the pin 44 a, 44 b can beextended or retracted therefrom, as described above. Further, the pins44 a, 44 b can be inserted into the workpiece 100 at slightly differentpositions so that one of the pins 44 a, 44 b “leads” the other pin 44 a,44 b along the predetermined path. In either case, the friction stirformed regions 114 can extend through the entire thickness of theworkpiece 100. The tools 40 a, 40 b can be used to plasticize successiveportions of the workpiece 100 until the entire workpiece 100 or onlyselect portions of the workpiece 100 have been processed. Also, while asingle workpiece 100 is illustrated in FIGS. 4 and 5, it is understoodthat the workpiece 100 can be formed of multiple members and that thefriction stir forming tools 40 a, 40 b can join the members by frictionstir welding.

As illustrated in FIG. 6, a forming apparatus 10 c can include twoopposed shoulders 42 c, 42 d that are defined by a single self-reactingfunction stir forming tool 40 c. The shoulders 42 c, 42 d are connectedby a pin 44 c which extends through the workpiece 100 so that the firstshoulder 42 c is directed toward the first side 102 of the workpiece 100and the second shoulder 42 d is directed toward the second side 104 ofthe workpiece 100. An actuator 50 c, similar to the actuators 50, 50 a,50 b described above, is connected to the friction stir forming tool 40c and configured to adjust the tool 40 c through the workpiece 100. Atthe start of a processing operation, the pin 44 c can be introduced intothe workpiece 100 from an end of the workpiece 100, or the pin 44 c canbe disconnected from one or both of the shoulders 42 c, 42 d so that thepin 44 c can be disposed through a hole in the workpiece 100 andassembled with the shoulders 42 c, 42 d as shown. The actuator 50 c canthen adjust the tool 40 c though the workpiece 100 along a nonplanarpath so that the rotating pin 44 c plasticizes portions of the workpiece100, and the shoulders 42 c, 42 d urge the workpiece 100 to a desired,nonplanar configuration.

After friction stir forming by any of the processes described above, theworkpiece 100 and any additional members 110 thereof can also beotherwise processed in the apparatus 10, 10 a, 10 b, 10 c. For example,the workpiece 100 can be machined to predetermined tolerances by amachining head (not shown), which can replace the friction stir formingtool 40, 40 a, 40 b, 40 c on the actuators 50, 50 a, 50 b, 50 c or canbe provided separately. Further, heat treatments can be performed byheating and cooling the workpiece 100 to one or more predeterminedtemperatures according to a predetermined schedule. The heattreatment(s) can be performed while the workpiece 100 is secured in theapparatus 10, 10 a, 10 b, 10 c, or the workpiece 100 can be removed fromthe apparatus 10, 10 a, 10 b, 10 c and heat treated in a differentdevice.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. For example, whilethe shoulders 42, 42 a, 42 b, 42 c, 42 d of the forming tools 40, 40 a,40 b, 40 c are used to urge the workpiece 100 to the desiredconfiguration in the foregoing examples, it is also appreciated that theworkpiece 100 can be bent, i.e., at least partially elastically, to thedesired shape and restrained in that shape while the forming tools 40,40 a, 40 b, 40 c process the workpiece 100 by friction stir forming,such that the workpiece 100 maintains the desired shape after beingreleased. The restraint can be accomplished using mechanical orhydraulic clamps or other restraining devices. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A method of forming a workpiece to a desired non-planar configurationwith a friction stir forming tool defining a shoulder and a pin, themethod comprising: providing a die having a non-planar contour surfacecorresponding to the desired configuration of the workpiece; supportingthe workpiece at least partially against the die; urging the shoulder ofthe forming tool against the workpiece and at least partiallypenetrating the workpiece with the pin such that the workpiece is bentagainst the contour surface; rotating the pin of the tool within theworkpiece at least partially during said urging step to plasticize aportion of the workpiece; and adjusting the tool in a predefined pathcorresponding to the contour surface of the die at least partiallyduring said urging step, such that the shoulder urges the workpiece tothe desired configuration and the tool forms a friction stir formedregion having a refined grain structure in the workpiece.
 2. A methodaccording to claim 1 further comprising providing the workpiece beingformed of at least one of the materials of the group consisting ofsteel, aluminum, titanium, and alloys thereof.
 3. A method according toclaim 1 wherein said providing step comprises providing a die defining adome-shaped contour surface and said adjusting step comprises moving thetool along the dome-shaped contour surface.
 4. A method according toclaim 1 wherein said urging step comprises urging the pin into theworkpiece through about half the thickness of the workpiece.
 5. A methodaccording to claim 1 wherein said adjusting step comprises successivelyplasticizing substantially an entire surface of the workpiece andthereby refining the grain structure of the workpiece over the surface.6. A method according to claim 1 wherein said adjusting step comprisesmaintaining the shoulder generally parallel to the contour surface.
 7. Amethod according to claim 1 wherein said providing step comprisesforming at least one detail feature on the contour surface of the diesuch that a corresponding detail feature is formed on the workpiece. 8.A method according to claim 1 wherein said supporting step comprisesproviding at least two structural members as the workpiece, thestructural members being joined during said adjusting step.
 9. A methodaccording to claim 1 further comprising rotating the workpiece inconjunction with said adjusting step.
 10. A method according to claim 1further comprising adjusting the pin relative to the shoulder.
 11. Amethod of forming a workpiece having first and second opposite sides toa desired, non-planar configuration, the method comprising: providing atleast one friction stir forming tool, the at least one tool definingfirst and second shoulders, the shoulders configured in an opposingconfiguration and directed generally inward with at least one pinextending at least partially therebetween; supporting the workpiecebetween the first and second shoulders, such that the first shoulder isdirected toward the first side of the workpiece and the second shoulderis directed toward the second side of the workpiece; urging the firstshoulder against the first side of the workpiece and the second shoulderagainst the second side of the workpiece; rotating the at least one pinwithin the workpiece at least partially during said urging step toplasticize a portion of the workpiece; and adjusting the at least onetool along a predefined path at least partially during said urging step,such that the shoulders urge the workpiece to the desired, non-planarconfiguration and the at least one pin forms a friction stir formedregion having a refined grain structure in the workpiece.
 12. A methodaccording to claim 11 wherein said providing step comprises providingfirst and second friction stir forming tools, the first tool definingthe first shoulder and the second tool defining the second shoulder,each tool defining one of the at least one pins extending therefrom. 13.A method according to claim 12 wherein said urging step comprisesinserting the pins of the first and second tools by a distance of abouthalf the thickness of the workpiece such that a friction stir formedregion formed in the first side of the workpiece extends to a frictionstir formed region formed in the second side of the workpiece.
 14. Amethod according to claim 11 wherein said providing step comprisesproviding the first and second shoulders connected by the pin extendingtherebetween.
 15. A method according to claim 11 further comprisingproviding the workpiece being formed of at least one of the materials ofthe group consisting of steel, aluminum, titanium, and alloys thereof.16. A method according to claim 11 wherein said adjusting step comprisesadjusting the tools through a curved path such that the shoulders urgethe workpiece to a curved configuration.
 17. A method according to claim11 wherein said adjusting step comprises successively plasticizingsubstantially the entire first and second sides of the workpiece andthereby refining the grain structure of the workpiece over the first andsecond sides.
 18. A method according to claim 11 wherein said adjustingstep comprises maintaining the shoulders in a generally parallelconfiguration.
 19. A method according to claim 11 wherein said adjustingstep comprises axially adjusting the at least one pin relative to theshoulder from which the pin extends.
 20. A method according to claim 11wherein said supporting step comprises providing at least two structuralmembers as the workpiece, the structural members being joined duringsaid adjusting step.
 21. An apparatus for forming a workpiece to adesired, non-planar configuration, the apparatus comprising: a diehaving a contour surface corresponding to the desired, non-planarconfiguration of the workpiece; a friction stir forming tool defining ashoulder and a rotatable pin extending from the shoulder, the tool beingconfigured to urge the shoulder and pin toward the contour surface ofthe die such that the shoulder urges the workpiece against the die andto the desired configuration, and the pin at least partially penetratesthe workpiece and rotates to form a friction stir formed region definedby a refined granular structure; and an actuator for adjusting thefriction stir forming tool, the actuator configured to adjust the toolalong a predefined path corresponding to the contour surface of the diesuch that the shoulder urges the workpiece to the desired, non-planarconfiguration.
 22. An apparatus according to claim 21 wherein thecontour surface of the die defines a dome and the actuator is configuredto adjust the tool along the dome defined by the die.
 23. An apparatusaccording to claim 21 wherein the pin is about half the thickness of theworkpiece.
 24. An apparatus according to claim 21 wherein the actuatoris configured to adjust the friction stir forming tool in at least twolinear directions and about at least two axes of rotation.
 25. Anapparatus according to claim 21 wherein the contour surface of the diedefines at least one detail feature.
 26. An apparatus according to claim21 wherein the die is configured to be rotated relative to the frictionstir forming tool.
 27. An apparatus for forming a workpiece to adesired, non-planar configuration, the apparatus comprising: at leastone friction stir forming tool, having first and second opposingshoulders and at least one pin extend at least partially between theshoulders, the shoulders being directed generally inward to receive aworkpiece therebetween; and at least one actuator for adjusting thefriction stir forming tool, the actuator being configured to adjust theshoulders in an inward direction such that the shoulders are urgedagainst opposite sides of the workpiece and rotating and at leastpartially penetrating the workpiece with the pin to form a friction stirformed region defined by a refined granular structure, wherein theactuator is configured to adjust the friction stir forming tool along apredefined path corresponding to the desired, non-planar configurationof the workpiece such that the shoulders urge the workpiece to thedesired, non-planar configuration.
 28. An apparatus according to claim27 wherein the pin extends from the first shoulder to the secondshoulder.
 29. An apparatus according to claim 27 wherein the formingtool has first and second pins, the first pin extending from the firstshoulder and the second pin extending from the second shoulder.
 30. Anapparatus according to claim 27 wherein the at least one actuator isconfigured to adjust the friction stir forming tool in at least twolinear directions and about at least two axes of rotation.
 31. Anapparatus according to claim 27 wherein the shoulders of the first andsecond friction stir forming machines are configured in a generallyparallel configuration.